Conventionally, when an ultrasonic image (for example, B mode image) is shot, transmission and reception beams are formed by setting transmission delay time and reception delay time using a sound velocity of a constant value under an assumption that the speed (sound velocity) of ultrasonic waves propagating in a living body in a scan range is constant. The sound velocity in the living body is, however, not necessarily constant and disturbance of the transmission and reception beams (disturbance of wavefronts) occurs and image quality deteriorates when the delay time is set based on the above-mentioned assumption.
For solving the above-mentioned problem, a method for estimating disturbance of wavefronts (wavefront distortion) using correlations among signals received by respective elements of an ultrasonic probe to correct the wavefront distortion has been developed. Furthermore, a method has been known for estimating and correcting wavefront distortion when plane waves at different deflection angles are transmitted and transmission wavefronts are synthesized based on reception signals received by respective elements to form a high-resolution image. In the above-mentioned methods, under an assumption that a layer (hereinafter, non-uniform layer) causing the sound velocity to be non-uniform in a propagation path of ultrasonic waves is in close contact with the ultrasonic probe, reception wavefront distortion is obtained from difference in delay time between reception channels and correction of the transmission wavefront distortion is repeatedly performed.
The non-uniform layer in the living body is not only in close contact with the ultrasonic probe but also it may present at a position distanced from the ultrasonic probe. In addition, the propagation path at the time of transmission is different from that at the time of reception. In the above-mentioned methods, an estimation result of the reception wavefront distortion is fed-back for correction of the transmission wavefront distortion on the different propagation path under the assumption that the non-uniform layer is in close contact with the ultrasonic probe. This means that the above-mentioned methods do not necessarily correct the wavefront distortion sufficiently.